1. Field of the Invention
The present invention relates to a novel dielectric ceramic composition suited as a material of resonators and circuit boards used in the microwave region mounted, for example, on mobile phones, cordless telephones, personal wireless telephony and satellite broadcast receivers, and to a dielectric resonator.
2. Description of the Prior Art
In recent years, dielectric ceramics have been widely used in the microwave region accompanying the realization of mobile phones, cordless telephones, personal wireless telephony and satellite broadcast receivers. The dielectric ceramics for microwave applications are chiefly used for the resonators requiring, however, the following three properties to be satisfied--(1) the dielectric ceramic should exhibit a large dielectric constant relative to its size that is urged to be reduced, since the wavelength is contracted to 1/.epsilon.r.sup.1/2 in the dielectric, (2) the dielectric ceramic should exhibit small dielectric loss at high frequencies, i.e., should have a high Q-value, and (3) the dielectric ceramic should exhibit a resonance frequency that changes little with a change in the temperature, i.e., should have a small temperature dependence of the dielectric constant and should remain stable.
Dielectric ceramics of this kind which have been known so far include oxide ceramic materials such as BaO-TiO.sub.2 type material, BaO-REO-TiO.sub.2 (where REO denotes an oxide of a rare earth element) type material, MgTiO.sub.3 -CaTiO.sub.3 type material and like materials (see, for example, Japanese Laid-Open Patent Publications Nos. 10806/1986, 100058/1988 and 19603/1985).
The BaO-TiO.sub.2 type material exhibits a dielectric constant .epsilon.r which is as great as 37 to 40 and a Q-value of as great as 40,000. It is, however, difficult to obtain the BaO-TiO.sub.2 type material which exhibits a temperature coefficient .tau.f of resonance frequency which is zero in a single phase. Moreover, the BaO-TiO.sub.2 type material permits the dielectric constant to greatly change with a change in the composition and further permits the dielectric constant to greatly change depending upon the temperature. With this material, therefore, it is difficult to stably decrease the temperature coefficient .tau.f of resonance frequency maintaining a large dielectric constant and a small dielectric loss.
As for the BaO-REO-TiO.sub.2 material, there has been known a BaO-Nd.sub.2 O.sub.3 -TiO.sub.2 type material or a BaO-Sm.sub.2 O.sub.3 -TiO.sub.2 type material. However, though these materials exhibit dielectric constants .epsilon.r of as very great as 40 to 60 and a temperature coefficient .tau.f of resonance frequency which is zero, their Q-values are as small as 5000 or less.
Moreover, the MgTiO.sub.3 -CaTiO.sub.3 type material exhibits a Q-value which is as great as 30,000 and a temperature coefficient .tau.f of resonance frequency which is zero, but exhibits a dielectric constant .epsilon.r which is as small as 16 to 25.
Thus, none of the above-mentioned materials fully satisfy the aforementioned three properties required for the dielectric material for high-frequency applications.
The present invention was contrived in view of the above-mentioned defects, and provides a dielectric ceramic composition which has a large dielectric constant, a large Q-value, small temperature dependence of the dielectric constant and remains stable, and a dielectric resonator.
In order to solve the above-mentioned problems, the present inventors have forwarded the study and have discovered that a dielectric ceramic composition could be obtained having a large dielectric constant, a large Q-value, small temperature dependence of the dielectric constant and which is stable if the composition is composed of Ln.sub.2 O.sub.x, Al.sub.2 O.sub.3, CaO, and TiO.sub.2 (Ln is at least one or more kinds of rare earth elements, and 3.ltoreq.x .ltoreq.4) which are adjusted to lie over a particular range.
That is, the dielectric ceramic composition of the present invention contains, as metal elements, a rare earth element (Ln) , Al, Ca and Ti. Here, when these components are expressed in terms of a molar ratio as aLn.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO. dTiO.sub.2, the values of a, b, c, d and x satisfy a+b+c+d=1, 0.056.ltoreq.a.ltoreq.0.214, 0.056 .ltoreq.b.ltoreq.0.213, 0.286.ltoreq.c.ltoreq.0.500, 0.230.ltoreq.d.ltoreq.0.470, and 3.ltoreq.x.ltoreq.4. Moreover, the dielectric resonator of the present invention comprises a dielectric ceramic disposed between a pair of input and output terminals and operates relying upon the electromagnetic coupling, wherein the dielectric ceramic contains, as metal elements, a rare earth element (Ln), Al, Ca and Ti. Here, when these components are expressed in terms of a molar ratio as aLn.sub.2 O.sub.x bAl.sub.2 O.sub.3 cCaO dTiO.sub.2, the values of a, b, c, d and x satisfy a+b+c+d=1, 0.056.ltoreq.a.ltoreq.0.214, 0.056.ltoreq.b.ltoreq.0.214, 0.286.ltoreq.c.ltoreq.0.500, 0.230.ltoreq.d.ltoreq. 0.470, and 3.ltoreq.x.times..ltoreq.4.
The dielectric ceramic composition of the present invention is a composite oxide containing a rare earth element (Ln) , Al, Ca and Ti as metal elements. Described below is the reason why the composition is limited within the above-mentioned ranges.
The range 0.056.ltoreq.a.ltoreq.0.214 is selected because of the reason that when 0.056 &gt;a, the temperature coefficient .tau.f becomes great having a positive sign and the absolute value of .tau.f exceeds 30. When a&gt;0.214, the dielectric constant decreases, the Q-value becomes smaller than 20,000, the temperature coefficient .tau.f becomes great having a negative sign and its absolute value exceeds 30. Particularly preferred range is 0.078.ltoreq.a.ltoreq.0.1166.
The range 0.056.ltoreq.b.ltoreq.0.214 is selected because when 0.056&gt;b, the Q-value becomes smaller than 20,000 and the temperature coefficient .tau.f increases having a positive sign. When b&gt;0.214, the Q-value becomes smaller than 20,000. Particularly preferred range is 0.078.ltoreq.b.ltoreq.0.1166.
Furthermore, the range 0.286.ltoreq.c.ltoreq.0.500 is selected because when 0.286&gt;c, the Q-value becomes smaller than 20,000 and when c&gt;0.500, the temperature coefficient .tau.f becomes great having the negative sign and its absolute value exceeds 30. Particularly preferred range is 0.390.ltoreq.c.ltoreq.0.47.
Moreover, the range 0.230&lt;d&lt;0.470 is selected because when 0.230.gtoreq.d, the temperature coefficient .tau.f becomes great having the negative sign and when d.gtoreq.0.470, the Q-value becomes smaller than 20,000. Particularly preferred range is 0.340.ltoreq.d.ltoreq.0.422.
Rare earth elements (Ln) may be Y, La, Ce, Pt, Sm, Eu, Gd, Dy, Er, Yb and Nd. Among them, Nd is most preferred. In the present invention, there may be used two or more kinds of rare earth elements (Ln). From the standpoint of temperature dependence of the dielectric constant, it is desired to use Y, Ce, Pr, Sm, Eu, Gd, Dy, Er and Yb.
A preferred dielectric ceramic has a composition expressed by the following formula: EQU aNd.sub.2 O.sub.3.bA.sub.2 O.sub.3.cCaO.dTiO.sub.2
wherein a, b, c, and d have the same meaning as mentioned above.
And another preferred ceramic has a composition expressed by the following formula: EQU ALm.sub.2 O.sub.x.bAl.sub.2 O.sub.3.cCaO.dTiO.sub.2
wherein Lm denotes a combination of Nd and other rare earth element selected from the group consisting of Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er and Yb, said Nd and other rare earth element being present at an atomic ratio of 9:1 to 1:9, especially 9:1 to 5:5.
According to the present invention, furthermore, the dielectric ceramic composition consists of the above-mentioned composition as main components and to which may be further added ZnO, NiO, SnO.sub.2, Co.sub.3 O.sub.4, MnCO.sub.3, ZrO.sub.2, WO.sub.3, LiCO.sub.3, Rb.sub.2 CO.sub.3, Sc.sub.2 O.sub.3, V.sub.2 O.sub.5, CuO, SiO.sub.2, MgCO.sub.3, Cr.sub.2 O.sub.3, B.sub.2 O.sub.3, GeO.sub.2, Sb.sub.2 O.sub.5, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, and the like compound. These compounds can be added in amounts of smaller than 6% by weight though it may vary depending upon the components that are added. Among them, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 that are added in amounts of 1 to 4% by weight help increase the dielectric constant compared with that of when they are not added, and help bring the temperature characteristics close to 0. It is therefore allowed to obtain a dielectric ceramic having excellent properties.
The dielectric ceramic composition of the present invention is prepared, for example, as described below. As starting materials, the powders of a rare earth oxide of a high purity, an aluminum oxide, a titanium oxide and a calcium carbide are weighed to be at desired ratios. Powders of Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, ZnO and the like may be added to the above main components. Thereafter, pure water is added thereto and the starting materials are mixed and pulverized under the wet condition by a mill using zirconia balls for 10 to 30 hours until the average particle diameter of the mixed components becomes smaller than 1.6 .mu.m. The mixture is dried, calcined at 1100.degree. to 1300.degree. C. for 1 to 4 hours, admixed with a binder in an amount of 0.8 to 5% by weight and are granulated. The obtained powder is molded into any desired shape by a molding means such as dry press, cold hydrostatic pressure press or extrusion molding, and is then fired in the open air at a temperature of 1400.degree. to 1700.degree. C. for 1 to 10 hours.
The dielectric resonator of the present invention, e.g., the TE-mode type resonator shown in FIG. 1 has an input terminal 2 and an output terminal 3 formed on both sides of a metal case 1, and has a dielectric ceramic 4 of the aforementioned composition which is disposed between these terminals 2 and 3. In this TE-mode type dielectric resonator, the microwaves are input through the input terminal 2 and are reflected by the boundary between the dielectric ceramic 4 and free space, and are confined in the dielectric ceramic 4 to develop resonance of a particular frequency. The signals are electromagnetically coupled to the output terminal 3 and are output. Though not diagramed, the dielectric ceramic composition of the present invention may be further adapted to a coaxial resonator of the TEM mode, a strip line resonator, a dielectric ceramic resonator of the TM mode and to any other resonators as a matter of course.
The dielectric ceramic composition of the present invention is a composite oxide containing a rare earth element (Ln), Al, Ca and Ti as metal elements. By adjusting these components within a particular range, it becomes possible to obtain a dielectric ceramic composition having a large dielectric constant, a large Q-value , small temperature dependence of the dielectric constant and which is stable.
By using the dielectric ceramic having the above-mentioned composition, furthermore, it is allowed to fabricate a dielectric resonator which operates upon the electromagnetic coupling featuring a small size, small loss, and stable resonance frequency at a temperature at which it is used.